Measurements of single particles trapped in air have been made since the pioneering work on electro-quasistatic levitation (Fletcher, H., “A Determination of Avogadro's Constant N from Measurements of the Brownian Movements of Small Oil Drops Suspended In Air,” Phys. Rev. 4, 440-453 (1914) and optical levitation (Ashkin, A., “Acceleration and Trapping of Particles by Radiation Pressure,” Phys. Rev. Lett. 24, 156-159 (1970). These techniques have been used to measure single-particle Raman spectra. The power of such Raman measurements is clearly recognized. However, the one reported attempt to measure single-particle Raman spectra of atmospheric aerosols achieved such a low sample rate that the authors wrote that the measurements were “tedious.” A problem in that approach is that charging of the aerosols was required. Only a fraction of the particles have the needed charge-to-mass ratio to be captured. Optical trapping using radiation pressure is useful for “suspending relatively transparent particles in relatively transparent media,” but it has not been used to sample and trap successively arriving particles from atmospheric air.
Previous attempts have been made using methods and/or systems for particle trapping using photophoretic force. However, the previous methods for trapping particles in air are generally capture and trap one single particle from a large group of particles (e.g., a few to 1000's) passively (a particle is randomly trapped). Furthermore, in those methods, these particles are initially placed on a substrate or in a container and then forced into the air in a short time to generate very high particle concentrations for trapping. Such passive particle trapping approaches have low efficiency, are not suitable for continuously sampling and trapping.
The existing on-line analytical systems (e.g. single-particle fluorescence spectrometer, mass spectrometer) generally concentrate and focus the aerosol into a localized jet, and then the particles flows into an interrogation region carried by the airflow, where the particles are analyzed one-by-one as they rapidly flow through (they are not trapped in stationary at any location). The flowing through systems cannot be used for the measurement that has very weak signal such as Raman scattering signal, and also cannot be used for observing time-revolution process.
The existing optical trapping techniques used to study the physical, chemical, or biological properties of one or a few representative particles in air generally capture and trap the particles from a large group of particles (e.g., a few to 1000's). The particles to be trapped typically have to be with similar properties and are initially placed on a substrate or in a container and then forced into the air in a short time to generate very high particle concentrations for trapping. Such low efficiency, passive particle trapping approaches are not adequate for continuously sampling and trapping for the successively arriving particles with different properties.
The prior art technology includes methods and apparatus where particles are randomly captured and/or trapped; such as when particles are spread out and randomly captured by entering a trap zone. Such processes are time-consuming and require the source of particles to be physically and chemically uniform; otherwise the trapped particles may not be representative of typical ones (due to the feature of randomization of the methods). This problem hinders instrumentation for continuously sampling and trapping aerosol particles that arrive successively in the trapping system from an air stream that likely have particles in different physical and chemical properties.